2 Observations and calibrations

The three clusters were observed in the same night (July 3, 1998) with the 1.5 m Danish telescope at ESO (La Silla). We employed an EFOSC camera equipped with a Loral/Lesser CCD detector C1W7 with $2052\times 2052$ pixels. The pixel size is 15 $\mu$m, corresponding to 0.39" on the sky, which provides a full field of $13'\times13'$.

In Fig. 1a is shown a full field V 15 min exposure of Terzan 9. Notice the remarkably patchy extinction in the region. We estimated from the field stars variations of $A_V\approx 1.5$ mag between high and low extinction patches. In Fig. 1b we show a cluster extraction showing the core region. In Fig. 2a we show a V image (3 min exposure) extraction for NGC 6139 where the populous nature of the cluster is evident. Finally, in Fig. 2b we provide a V extraction (1 min exposure) of NGC 6453.

  

Figure 1: V image extractions of Terzan 9: a) whole field ($13'\times13')$; b) extraction of $1.6'\times1.6'$ showing the cluster post-collapse core structure. North is to the top and East to the left

The log of observations is provided in Table 1.

Daophot II was used to extract the instrumental magnitudes. For calibrations we used stars from Landolt (1983) and Landolt (1992).

Reduction procedures in such reddened crowded fields were discussed in detail in a study of Liller 1 (Ortolani et al. (1996) and references therein). The equations for the present clusters are:

V = 24.24 + v

I = 23.09 + i

reduced to 1 s exposure time and 1.1 airmass. Due to crowding effects arising in the transfer of the aperture magnitudes from standards to the field stars, the zero point calibration errors are dominant, estimated to be about $\pm 0.03$ mag. The CCD shutter time uncertainty (0.3 s) for a typical 10 s exposure time for the standard stars, produces an additional 3% uncertainty, which is propagated to the calibrations of the long exposure cluster frames. The final magnitude zero point uncertainty amounts to $\pm 0.05$.The atmospheric extinction was corrected with the La Silla coefficients (C_V=0.16, C_I=0.12 mag/airmass). The zero point of the V filter was checked relative to Hazen-Liller (1984) for NGC 6453, while that for NGC 6139 was done relative to Zinn & Barnes (1998). Three bright standard stars from Hazen-Liller, in common to our field, give a difference $\Delta$(Hazen-present) = 0.015. Three bright stars in common to Zinn & Barnes provide a difference $\Delta V{\rm (Zinn-present)}=0.01$ $\pm 0.1$ and $\Delta(V-I){\rm (Zinn-present)}=0.01 \pm 0.2$. There is agreement between the present and previous calibrations, with no systematic trend.

  

Figure 2: V image extractions of $1000\times 1000$ pixels ($6.5'\times6.5'$)for: a) NGC 6139; b) NGC 6453. Orientation as in Fig. 1

  

Table 1: Log of observations